I have a large layer 2 network with quite a lot of vlans and hosts , they all aggregate to a pair of 3750s (stacked) which are doing the layer 3 routing and are the default gateway of all the vlans
i need to migrate from these 3750s onto new kit on which i will configure the ips that the old 3750 stacks used to have.
I dont really want too much downtime for this , assuming my old 3750 stack goes down , what is the average time hosts will hold the old arp entries for the default gateways? the hosts are made up of quite a variety of things , mainly windows servers, and can anyone think of a better of doing this than shutting down my old switches and reconfiguring my new kit to take the old role.
i thought of enabling hsrp , but i will still get same problem as above when i configure hsrp on the old 3750s.
Using the command clear arp-cache forces a Cisco router/multilayer switch forces it to clear the ARP cache on its interfaces (not needed for your purpose), and in addition, to send gratuitous ARP replies on all cleared Layer3 interfaces. This will enable the stations to refresh their ARP caches.
You may select a combination of these approaches:
- Introduce the new 3750 without using HSRP and use the clear arp-cache command on the new switch to send gratuitous ARP replies
- Configure HSRP on the old 3750 and use the clear arp-cache to make stations learn the new vIP/vMAC mapping. Then introduce the new 3750 using the HSRP and simply perform active/standby router switchover.
Relying on ARP cache expiration is not reliable - it can take minutes, and it depends very much on the operating system. I believe that Linux and Windows expire the entries after around 60 seconds.